1. Field of the Invention
The present invention relates to a surface light source device, a diffusion plate and a liquid crystal display device, and particularly to a surface light source device used as a back light for a liquid crystal display panel or the like, a diffusion plate for the surface light source device and a liquid crystal display device using the surface light source device.
2. Description of the Related Art
A surface light source device is used as a back light or the like for a transmission type liquid crystal display panel. The liquid crystal panel generates an image by transmitting or intercepting light every pixel. The liquid crystal display panel has no function of spontaneously emitting light, and thus it needs a surface light source device for back light.
FIG. 1 is an exploded perspective view showing the structure of a conventional surface light source device 1, and FIG. 2 is a cross-sectional view of FIG. 1.
The surface light source device 1 comprises a light guide plate 2 for confining light therein, a light emitting portion 3 and a reflection plate 4. The optical guide plate 2 is formed of transparent resin having a high refractive index such as polycarbonate resin, methacrylic resin or the like, and a diffusion pattern 5 is formed on the back surface of the optical guide plate 2 by using irregularities processing, dot printing of diffusing/reflecting ink or the like. The light emitting portion 3 includes plural light emission diodes (LED) 7 mounted on the front surface of a circuit board 6, and the light emission diodes 7 are arranged so as to face a side surface (light incident side surface 8) of the light guide plate 2. The reflection plate 4 is formed of a white resin sheet having high reflectivity, for example, and it is attached to the lower surface of the light guide plate 2 by double-sided tape.
In the surface light source device 1, as shown in FIG. 2, light p emitted from the light emitting portion 3 and guided from the light incident side surface 8 into the light guide plate 2 is repetitively totally reflected between the obverse and back surfaces of the light guide plate 2 and propagates in such a direction as to be far away from the light emitting portion 3 while being confined in the light guide plate 2. The light p propagating through the light guide plate 2 as described above is incident to the back surface of the light guide plate 2, and diffused/reflected by a diffusion pattern 5. At this time, a part of the light p that is reflected from the diffusion pattern 5 formed on the lower surface of the light guide plate 10 to the surface (light emission face 10) of the light guide plate 2 at an angle smaller than the critical angle of the total reflection is emitted from the light emission face 10 to the outside of the light guide plate 2. On the other hand, a part of the light p that passes through a portion of the lower surface of the light guide plate 2 on which no diffusion pattern 5 is formed and emits from the back surface of the light guide plate 2 is reflected from the reflection plate 4, returned into the inside of the light guide plate 2 and confined in the light guide plate 2 again. Accordingly, the loss of light amount from the back surface of the light guide plate 2 can be prevented by the reflection plate 4.
The light emitted from the light emission face 10 of the light guide plate 2 as described above is emitted to from a medium having a large refractivity to a medium having a small refractivity, and thus the light is emitted with being close to the light emission face 10, as shown in FIG. 3. Assuming that the x-axis is set along the width direction of the light incident side surface 8, the y-axis is set along the direction perpendicular to the light incident side surface 8 and the z-axis is set along the direction perpendicular to the light emission face 10, the light emitted from the light emission face 10 has a slender directivity profile extending substantially in the y-axis. Under this state, the light emission face 10 of the surface light source device 1 looks dark when viewed from the direction (z-axis direction) perpendicular to the light emission face 10. Therefore, it is general that the peak direction of the directivity profile of light is oriented to the z-axis direction perpendicular to the light emission face 10 by disposing a diffusion plate 11 having relatively large diffusion degree on the light emission face 10 and diffusing light emitted from the light emission face 10 with the diffusion plate 11 as shown in FIG. 3.
Besides, a prism sheet 13 is used as shown in FIG. 4 when stronger directivity is needed as compared with the case of FIG. 3. That is, the prism sheet 13 is disposed above the light emission face 10 of the light guide plate 2, and diffusion plates 12, 14 are disposed at the back and obverse sides of the prism sheet 13, respectively. In this case, light emitted from the light emission face 10 is diffused by the diffusion plate 12 so that the direction of light directivity is approached to the vertical direction, and then oriented to the vertical direction by the prism sheet 13. Thereafter, the light is further diffused by the diffusion plate 14 so as to emit in the direction perpendicular to the light emission face 10. Here, the action of the diffusion plate 12 is to make light incident to the prism sheet 13 at such an angle that light passing through the prism sheet 13 directs in the z-axis direction. As shown in FIG. 4, there exists an emission angle α at which light is hardly emitted from the prism sheet 13 in an oblique direction to the vertical direction (i.e., the light intensity of light passing through the prism sheet 13 is minimum at the angle α). When viewed from the direction of the angle α, images on the liquid crystal display panel are hardly viewed. Therefore, the light is diffused by the diffusion plate 14 so that a part of diffused light is distributed in the direction of the angle a and thus an image can be viewed over a broad range with the z-axis direction at the center of the range. Furthermore, the diffusion plates 12, 14 and the prism sheet 13 also have a function of shielding the diffusion pattern 5 formed on the lower surface of the light guide plate 2 so that the diffusion pattern 5 is hidden from view from the front side.
The power consumption is more greatly reduced in the case of the surface light source device using LED as described above than in the case of the surface light source device using a cold cathode ray tube. However, the surface light source device using LED is used for commercial products having high portability like portable information terminals such as cellular phones, PDA (Personal Digital Assistance), etc. from the viewpoint of performance of compactness in size and lightness in weight, and increase of the lifetime of power sources is strongly required for these products to enhance convenience when they are carried, and also reduction of the power consumption is required. Accordingly, reduction in power consumption is also strongly required for the surface light device (back light) used in these commercial products. Therefore, LED having higher efficiency is used in the surface light source device, and as light emission efficiency of light emitting elements is enhanced, the number of light emitting elements to be used is reduced.
However, in the case of the surface light source device 1 equipped with the light emitting portion 3 having plural LEDs 7 which are arranged in a line to be designed as a linear light source as shown in FIG. 1, if the number of LEDs 7 is reduced, the light emission face (light emission face) becomes dark or luminance unevenness (unevenness in brightness) is intensified. Therefore, there is a limit to the reduction of the number of LEDs 7, and thus there is also a limit to the reduction in power consumption.
FIG. 5 shows a surface light source device 21 having a light emitting portion 23 in which several (preferably, one) light emitting elements such as LEDs or the like are collected in one place to thereby achieve a point light source.
In the surface light source device 21, the light emitting portion 23 designed in the form of a point light source is disposed so as to face a side surface (light incident face 22a) of a light guide plate 22 formed of transparent resin having a high refractive index such as polycarbonate resin, methacrylic resin or the like. On the lower surface of the light guide plate 22, a number of diffusion patterns 24 are arranged on arcs which are arranged concentrically with the light emitting portion 23 at the center. Each diffusion pattern 24 is formed in the recess shape on the lower surface of the light guide plate 22 to have an arcuate section, and it extends along the peripheral direction of the arcs arranged concentrically with the light emitting portion 23 at the center. The reflection face of each diffusion pattern 24 is perpendicular to the direction connecting the light emitting portion 23 and the diffusion pattern 24 concerned (this direction is assumed as “r-axis direction”) in plan view. The diffusion pattern 24 is formed so that the pattern density is gradually increased as the diffusion pattern 24 is remoter from the light emitting portion 23.
In the surface light source device 21, when the light emitting portion 23 is actuated to emit light, light emitted from the light emitting portion 23 is incident from the light incident face 22a into the light guide plate 22 and repetitively totally-reflected between the upper and lower surfaces of the light guide plate 22 while propagating from the light emitting portion 23 to a remoter side. Light diffused and reflected on the lower surface of the light guide plate 22 by the diffusion patterns 24 while propagating in the light guide plate 22 would be emitted from the upper surface of the light guide plate 22 (light emission face) if it is incident to the upper surface of the light guide plate 22 at an incident angle smaller than the critical angle of total reflection. However, in such a surface light source device 21 as described above, the light diffused and reflected by the diffusion patterns 24 is diffused on the zr plane, however, it is not diffused on the xy plane. When viewed from the z-axis direction, the light goes straight even after it is reflected by the diffusion patterns 24. Therefore, the amount of light emitted in any direction around the light emitting portion 23 is not varied even when the light is diffused by the diffusion patterns 24, and the amount of light transmitted in each direction in the light guide plate 22 is determined by the amount of light emitted in each direction from the light emitting portion 23.
According to the surface light source device 21 as described above, light can be uniformly emitted from the overall light emission face (i.e., the overall, light emission face can uniformly shine) by making light incident from the light emitting portion 23 in each direction in the light guide plate 22 so that the amount of the light corresponds to the distance at which the light passes through the light guide plate 22. By combining a transmission-type liquid crystal display panel with the surface light source device 21 as described above, a liquid crystal display device in which images are easily viewable at a wide angle can be manufactured, and also it can contribute to the reduction of the power consumption of the liquid crystal display device.
When there is a probability that plural persons view a screen at the same time like a note-type personal computer or the like, it is required to make the screen viewable at a wide angle, and thus a surface light source device for emitting light having broad directivity is needed. However, in the case of mobile equipment which is represented by a cellular phone, it is based on the assumption of personal use. Accordingly, it is preferable that the directivity is narrowed to make the screen invisible to neighbors in a train or the like. There is particularly required such a surface light source device that no light is emitted in oblique directions.
Furthermore, it is better that no light is emitted in the oblique directions because there is no extra emission light and thus the power consumption can be more greatly reduced. Or, the light emitted from the light emitting portion can be collected to the front side to enhance the brightness at the front side. At any rate, the efficiency (=brightness/power consumption) of the surface light source device can be enhanced.
However, when a diffusion plate is used as in the case of the surface light source device as the first prior art, light is emitted in all the directions, and thus it is difficult to narrow the emission range of light emitted from the liquid crystal display device.
Furthermore, in the case of the surface light source device 21 as the second prior art, the travel direction of light emitted from the light emitting portion 23 is directly changed to the direction perpendicular to the light guide plate 22 while the light is spread to the overall area in the light guide plate 22, thereby emitting the light from the light emission face. Therefore, this prior art has no device for narrowing the angle of beam spread (hereinafter referred to as “directivity angle”) of light.
In the case of the surface light source device 21 as the second prior art, radial luminance unevenness (emission lines) R around the light emitting portion 23 as shown in 6 is partially viewed when viewed from the oblique upper side. Therefore, when the surface light source device 21 is used in a liquid crystal display device or the like, the luminance unevenness R obstructs the view of images from some viewing directions, so that the quality of the image display device or the like is lowered.